The present invention relates to gas sensors, and in particular to gas sensing devices that detect the presence of a specific gas by monitoring the absorption of optical radiation transmitted through a chamber containing a sample of gas under test.
Gas sensors utilising an infra-red source and a corresponding infra-red detector are well known, in particular in the design of, for example, carbon dioxide and hydrocarbon gas detectors. Infra-red radiation emitted by the source is focussed onto the detector, having passed through a chamber containing the gas under test, where some of the infra-red radiation will be absorbed by the gas. The absorption by a specific gas is a function of the wavelength of the infra-red radiation, and by careful selection of an appropriate optical band-pass filter at the detector, it is possible to determine the presence of a specific gas.
A particularly important aspect of the design of optical absorption gas sensors is the path length between source and detector. In many known systems, long path lengths are used to increase the degree of absorption observed, typically by mounting the infra-red source and detector in separate housings for remote location from one another using long tubes or free space therebetween to define the optical path between source and detector. Such systems typically require pumping of gas through the detection chamber.
There is, however, a commercial requirement for highly compact, integral sensors, that can be simply plugged into, for example, portable gas detection units. This severely compromises the absorption path length available. In one compact sensor, as described in GB 2316172, a design of sensor attempts to increase path length between source and detector in a common housing and maximise signal to noise ratio characteristics by arranging the source and sensor at respective foci of an ellipsoidal chamber in which the light traveling between source and sensor is reflected at least three times. The sensor described requires high quality, polished focussing curved surfaces and careful positioning of the source and sensor elements.
The present invention aims to provide a highly compact gas sensor that is easy and cost effective to manufacture and assemble that avoids the need for focussing surfaces and careful positioning of source or detector elements.
According to one aspect, the present invention provides a gas sensor comprising:
an optical source for emitting radiation therefrom;
a detector sensitive to radiation emitted from the source;
a circumferential chamber, having optically reflective surfaces, extending between the source and the detector.
According to another aspect, the present invention provides a gas sensor comprising:
an optical source for emitting radiation therefrom;
a detector sensitive to radiation emitted from the source;
a chamber, extending between the source and the detector, defined by a plurality of non-focussing, planar surfaces disposed to form a folded optical pathway that includes a plurality of segments substantially parallel to one another.